[aspnetcore] Fix nullable enum and duplicate model

[yilinjuang]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

Generate enum as nullable only if isNullable is true. Remove generation of sub-enum in class if enum is model which is already generated in another file.

@wing328
@jimschubert

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[wing328]

LGTM

[wing328]

@frankyjuang thanks for the PR, which has been included in the v4.1.1 release: https://twitter.com/oas_generator/status/1165944867391860737

[edag94]

Thanks for this PR! Will be closing the issue I raised and link it to this

[wing328]

@frankyjuang when you've time next week, please kindly PM me via Slack